The present invention relates to a semiconductor device having thin-film transistors (TFTs) formed on an insulating substrate made of glass or the like and also to a method of fabricating such a semiconductor device.
Known semiconductor devices having TFTs on an insulating substrate made of glass or the like include active-matrix liquid-crystal displays and image sensors which use such TFTs to activate pixels.
Generally, TFTs used in these devices are made of a silicon semiconductor in the form of a thin film. Silicon semi-conductors in the form of a thin film are roughly classified into amorphous silicon semiconductors (a-Si) and crystalline silicon semiconductors. Amorphous silicon semiconductors are fabricated at low temperatures. In addition, they are relatively easy to manufacture by chemical vapor deposition. Furthermore, they can be easily mass-produced. Therefore, amorphous silicon semiconductors have enjoyed the widest acceptance. However, their physical properties such as conductivity are inferior to those of crystalline silicon semiconductors. In order to obtain higher-speed characteristics from amorphous silicon semiconductors, a method of fabricating TFTs consisting of a crystalline silicon semiconductor must be established and has been keenly sought for. It is known that crystalline silicon semiconductors include polysilicon, silicon crystallites, amorphous silicon containing crystalline components, and semi-amorphous silicon that is midway in nature between crystalline state and amorphous state.
Known methods of obtaining these crystalline thin-film silicon semiconductors include:
(1) During fabrication, a crystalline film is directly created.
(2) An amorphous semiconductor film is once formed. Then, the film is irradiated with laser light so that the energy of the laser light imparts crystallinity to the film.
(3) An amorphous semiconductor film is once formed. Thermal energy is applied to the film to crystallize it.
Where the method (1) above is utilized, it is technically difficult to form a semiconductor film having good physical properties over the whole surface uniformly. Also, the film is formed at a high temperature of over 600xc2x0 C. and so cheap glass substrates cannot be used. Hence, this method presents problems regarding costs.
In the method (2), an excimer laser is used most commonly today. If this excimer laser is employed, the laser light illuminates only a small area and hence the throughput is low. Furthermore, the stability of the laser is not stable enough to uniformly process the whole surface of a large-area substrate. Therefore, we feel that this method is a technique of the next generation.
The method (3) above can process substrates of larger areas compared with the methods (1) and (2). However, a high temperature exceeding 600xc2x0 C. is also necessary. It is necessary to lower the heating temperature where cheap glass substrates are used. Especially, liquid crystal displays having larger areas have tended to be manufactured today. With this trend, larger glass substrates have to be employed. Where larger glass substrates are used in this way, shrinkage and stress produced during a heating step that is essential for semiconductor fabrication deteriorate the accuracies of mask alignment and other steps. This presents serious problems. Especially, in the case of Corning 7059 which is most commonly used today, the strain point is 593xc2x0 C. Therefore, if the prior art heating-and-crystallization step is effected, a large distortion is induced. Besides the problem of temperature, the heating time, i.e., the time required for crystallization, poses problems. In particular, the heating time necessary for crystallization is as long as tens of hours or longer in the present process. Therefore, it is necessary to shorten the heating time.
It is an object of the present invention to provide means for solving the foregoing problems.
It is a more specific object of the invention to provide a method of fabricating a thin film of crystalline silicon semiconductor by forming a thin film of amorphous silicon and heating this film at a lower temperature and in a shorter time than heretofore to crystallize it.
Of course, a crystalline silicon semiconductor fabricated by the manufacturing process according to the invention has physical properties comparable or superior to the physical properties of crystalline silicon semiconductor devices fabricated by the prior art techniques and can be used in active layer regions of TFTs.
We formed amorphous silicon semiconductor films as described above by CVD processes and sputtering processes. These films were heated to crystallize them. We conducted experiments on this method of heating amorphous silicon semiconductor films and discussed the method as follows.
As an experiment, an amorphous silicon film was formed on a glass substrate. This film was crystallized by heating. We discussed the mechanism by which the film was heated and crystallized. Crystals began to grow at the interface between the glass substrate and the amorphous silicon. We observed that where a given film thickness was exceeded, the crystals grew like columns vertical to the substrate surface.
We understand the above-described phenomenon as follows. Crystal nuclei, or seed crystals, exist at the interface between the glass substrate and the amorphous silicon film, and crystals grow from these nuclei. We consider that these crystal nuclei are trace amounts of impurity metal elements existing on the surface of the substrate and the crystalline component of the glass surface. It is considered that crystalline component of silicon oxide (known as crystallized glass) is present on the surface of the glass surface.
Accordingly, we have thought that the crystallization temperature might be lowered by introducing crystal nuclei more positively. To confirm the effects of this temperature drop, we conducted an experiment. That is, a trace amount of other metal was deposited on a substrate. A thin film of amorphous silicon was formed on the metal layer. Then, the amorphous silicon was heated and crystallized. Where some metals are deposited on the substrate, crystallization temperature drop was confirmed. We imagined that crystals were growing from crystal nuclei of foreign substances. We further investigated the mechanism on plural impurity metals which permitted temperature decreases.
A crystallization process can be classified into two phases, i.e., creation of nuclei at the initial stage and crystal growth from the nuclei. The speed of the creation of nuclei at the initial stage can be known by measuring the time taken until microscopic dot-like crystals are created at a constant temperature. Where any of the above-described impurity metals was deposited as a thin film, the time was shortened. This demonstrates that introduction of crystal nuclei lowers the crystallization temperature. We discovered an unforeseen fact. Specifically, the growth of crystal grains subsequent to nucleation was investigated while varying the heating time. Where some metal was deposited as a film and then a thin film of amorphous silicon formed on the metal film was crystallized, crystals grew at an amazing rate after the nucleation. The mechanism of this phenomenon will be described in greater detail later.
In any case, we have discovered that if a trace amount of some metal is deposited as a film, a thin film of amorphous silicon is formed on the metal film, and then the amorphous silicon film is heated and crystallized, then sufficient crystallization is caused by the above-described two effects at a temperature lower than 580xc2x0 C. in a time of about 4 hours, which would have never been conceived heretofore. The material which showed the most conspicuous effects and we have selected out of impurity metals exhibiting such effects is nickel.
We now give examples of structure, illustrating the effect of nickel. A substrate made of Corning 7059 was not treated at all. That is, a thin film consisting of a trace amount of nickel was not formed on the substrate. A thin film of amorphous silicon was formed on the substrate by plasma CVD. This thin film was heated in a nitrogen ambient to crystallize the film. Where the heating temperature was 600xc2x0 C., the required heating time was 10 hours or longer. Where a thin film consisting of a trace amount of nickel was formed on the substrate, similar crystallization was induced by heating the thin film of amorphous silicon for about 4 hours. The crystallization was investigated by Raman spectroscopy. This demonstrates that the nickel produces very great effects.
As can be understood from the above description, where a thin film of amorphous silicon is formed on a thin film consisting of a trace amount of nickel, the crystallization temperature can be lowered. Also, the crystallization time can be shortened. It is assumed that this process is applied to fabrication of TFTs. We now describe the process in further detail.
Methods of implementing the addition of traces of nickel are first described. In a first method, a thin film is formed out of a trace amount of nickel on a substrate and then a film of an amorphous silicon is formed. In a second method, a film of amorphous silicon is first formed and then a thin film is formed out of a trace amount of nickel on the amorphous silicon film. Both methods can lower the temperature similarly. We have found that films can be formed by either sputtering or evaporation. That is, the process does not depend on the method of forming the films. Where a trace amount of nickel is deposited as a thin film on a substrate, a method consisting of forming a thin film of silicon oxide on a glass substrate of Corning 7059 and forming a thin nickel film out of a trace amount of nickel on the silicon oxide film produces greater effects than does a method of directly depositing a trace amount of nickel as a thin film on the substrate. We consider that the fact that silicon and nickel are in direct contact with each other is important for the temperature decrease, and that in the case of Corning 7059, components other than silicon may impede contact between silicon and nickel or reaction between them.
One method of adding a trace amount of nickel is to form a thin film in contact with the top or bottom surface of an amorphous silicon layer. We have confirmed that similar effects are produced where nickel is added by ion implantation, and that where the dopant concentration of nickel was in excess of 1xc3x971015 atoms/cm3, the temperature was lowered. Where the dopant concentration was greater than 1xc3x971021 atoms/cm3, the shape of the peak of the obtained Raman spectrum was distinctly different from the shape of the peak of the Raman spectrum obtained from a single substance of silicon. Therefore, we consider that the usable dopant concentration range is between 1xc3x971015 and 5xc3x971019 atoms/cm3. Where the thin film is used as the active layers of TFTs, taking account of the physical properties of the semiconductor, it is necessary to restrict the dopant concentration to the range from 1xc3x971015 to 1xc3x971019 atoms/cm3. The growth of crystals to which a trace amount of nickel is added and the features of the crystal morphology are described next. Also, the crystallizing mechanism estimated from these features is described.
Where nickel is not added, nuclei are created at random from crystal nuclei existing at the interface with the substrate. Also, crystals grow at random from the nuclei. It has been reported that crystals relatively well oriented in a (110) or (111) direction are obtained, depending on the method of fabrication. Of course, a substantially uniform crystal growth is observed over the whole thin film.
In order to confirm this mechanism, we made an analysis, using a differential scanning calorimeter (DSC). A thin film of amorphous silicon was formed on a substrate by plasma-assisted chemical vapor deposition (PCVD). The thin film was loaded into a container together with the substrate. The temperature was elevated at a constant rate. A clear heat-generating peak was observed in the neighborhood of 700xc2x0 C. Of course, this temperature was shifted with the temperature elevation rate. Where the rate was 10xc2x0 C./min, crystallization started at 700.9xc2x0 C. Then, measurements were made with three different temperature elevation rates. The activation energy for crystal growth after initial nucleation was found by the Ozawa""s method. The energy was about 3.04 eV. The reaction rate formula was compared with the theoretical curve to determine whether the formula fitted the curve. We have found that random creation of nuclei and its growth model can account for the activation energy best. This proves the validity of the theory that seed crystals are created at random from crystal nuclei existing at the interface with the substrate and then crystals grow from the nuclei.
Similar measurements were made except that a trace amount of nickel was added. Where the temperature was elevated at a rate of 10xc2x0 C./min, crystallization was started at 619.9xc2x0 C. The activation energy for crystal growth found from a series of measurements was approximately 1.87 eV. This numerical value demonstrates that crystal growth is promoted. The reaction rate formula found by the comparison with the theoretical curve approximated the one-dimensional interface reaction rate rule model. This suggests that crystals are grown in a certain direction. The data obtained from the above-described thermal analysis is listed in Table 1 below. The activation energy given in this Table 1 was found by measuring the quantity of heat released from each sample during heating of the sample and calculating the energy from the quantity of heat by analyzing means called the Ozawa""s method.
The activation energy given in Table 1 above is a parameter indicating the degree of easiness of crystallization. As the value of the activation energy is increased, it is more difficult to induce crystallization. Conversely, as the value is reduced, it is easier to induce crystallization. It can be seen from Table 1 that the activation energy of each sample containing nickel drops as crystallization progresses. That is, as crystallization progresses, crystallization is caused more easily. In the case of a crystalline silicon film formed by the prior art method without adding nickel, as crystallization progresses, the activation energy is increased. This indicates that as crystallization proceeds, it is more difficult to induce crystallization. Comparison of the average values of activation energy reveals that the value of the silicon film crystallized with addition of nickel is about 62% of the value of the silicon film crystallized without adding nickel. This indicates that an amorphous silicon film doped with nickel can be easily crystallized.
The morphologies of crystals to which nickel was added were observed with a transmission electron microscope. The results of the observation show that a region doped with nickel differs in crystal growth from adjacent regions. Specifically, a cross section of the nickel-doped region was observed. Moire fringes or other fringes which seemed to be a lattice image were substantially vertical to the substrate. We consider that the added nickel or its compound with silicon forms crystal nuclei which induced growth of columnar crystals substantially vertical to the substrate, in the same way as in the case where no nickel is added. In regions surrounding the nickel-doped region, crystals were observed to have grown like styli or columns parallel to the substrate.
The morphologies of crystals close to the nickel-doped regions were observed. First of all it was not expected that regions to which the trace amount of nickel was not directly added were crystallized. The concentrations of nickel in the region to which the trace amount of nickel was added, in lateral crystal growth regions close to the nickel-doped region, and in remoter amorphous regions were measured by secondary ion mass spectrometry (SIMS). At locations considerably remote from the nickel-doped region, low-temperature crystallization did not take place, and an amorphous region remained. As shown in FIG. 4, the nickel concentration in the lateral crystal growth regions was lower than the concentration in the nickel-doped region. The concentration in the amorphous regions was still lower by about 1 order of magnitude. That is, nickel atoms were diffused over a considerably broad region. In particular, the nickel concentration is high in the region in which nickel has been directly added. The lateral growth portion (the portion in which the crystal has grown parallel to the substrate) has a lower nickel concentration than the region in which nickel has been directly added.
It was observed from the TEM image of a surface close to the nickel-doped region that the greatest lateral crystals parallel to the substrate grew as long as hundreds of micrometers from the nickel-doped region, and that the amount of growth increases with the lapse of time and with elevating the temperature. As an example, growth of about 20 xcexcm was observed in a process conducted at 550xc2x0 C. for 4 hours. It was confirmed that this crystal growth proceeded in the form of stylus or column, and that the terminal portion (the front end) of the crystal growth contains nickel concentratedly. Spacial distribution of Ni was measured by EDX concerning columnar crystal which is characteristic of lateral growth, and examined correlation between the distribution and the columnar crystal. EDX measurement of the front end of Si was carried out. The result is shown in FIG. 10(A). FIG. 10(B) shows a measurement of a film containing no Ni for reference, and it can be considered that FIG. 10(B) indicates the lower limit of detection. Comparison of these two indicates that the front end contains a large amount of Ni.
The results of the experiments obtained as described above has led us to consider that crystallization progresses by the mechanism described now. First, crystal nuclei are created. The activation energy is reduced by the addition of a trace amount of nickel because the addition of nickel enables crystallization at lower temperatures. We consider that one reason is that nickel acts as a foreign substance. Another reason might arise from the fact that one of nickel-silicon intermetallic compounds has a lattice constant close to that of crystalline silicon. Every nucleation occurs almost simultaneously over the whole surface of the nickel-doped region. As a result, crystals grow while maintaining planes. In this case, the reaction rate formula is given by a one-dimensional interface reaction rate rule process. Thus, columnar crystals substantially vertical to the substrate are obtained. However, completely aligned crystallographic axes cannot be derived because of the restriction imposed by the film thickness and because of the effects of stress or the like.
The crystal components parallel to the substrate are more uniform than components vertical to the substrate. Therefore, column- or stylus-like crystals grow uniformly laterally around crystal nuclei created in the nickel-doped region. Of course, it is expected that the reaction rate formula is given by a one-dimensional interface reaction rate rule process. Since the activation energy for crystal growth is reduced by the addition of nickel as described previously, it is expected that the lateral growth rate is very high and in fact this is true.
The electrical characteristics of the nickel-doped region and of the nearby lateral growth regions are described next. Of the electrical characteristics of the nickel-doped region, the conductivity is approximate to that of a film to which almost no nickel is added. This film was crystallized at about 600xc2x0 C. for tens of hours. The activation energy was found from the temperature-dependence of the conductivity. Where the nickel concentration was 1017 to 1018 atoms/cm3, any behavior which seems to be contributable to the energy levels of nickel was not observed. That is, these experimental results have led us to consider that the nickel-doped region can be used as the active layers of TFTs if this region has above-described concentration. The experimental results are shown in FIG. 9. The sample used in the experiment is prepared as follows. Corning 7059 glass is used for substrate. SiO2 base 2000 xc3x85 film is formed by sputtering on the glass. Then an amorphous silicon film is formed with SiH4/H2 mixed gas by CVD, and thereafter a small quantity of Ni is added by plasma treatment utilizing Ni electrode. The treatment conditions are as follows.
Thermal crystallization was carried out at between 450xc2x0 C. and 700xc2x0 C. after 1 hour hydrogen extraction at 430xc2x0 C. The atmosphere in which crystallization was carried out was nitrogen atmosphere. Nitrogen flowed in and out. We examined electric characteristics (conductivity) of crystalline silicon semiconductor by measuring temperature dependency which was measured on electric current and voltage by coplanar-type Al electrode which was formed on the silicon film. Activation energy of FIG. 9 is obtained from the conductivity. It can be said that the value of activation energy is appropriate as crystalline silicon semiconductor as far as our experiments are concerned, and effects on electric characteristics (conductivity) by the energy levels of Ni are very small at least when measured at around normal temperature.
On the other hand, the conductivity of the lateral growth portions is higher than that of the nickel-doped region by at least one order of magnitude and is comparatively high for a crystalline silicon semiconductor. Since the direction of flow of electrical current agrees with the direction of lateral crystal growth, we consider that grain boundaries to hinder the movement of electrons do not or hardly exist between the electrodes. This agrees well with the results of the TEM images. That is, carriers are moved along the grain boundaries of crystals grown like styli or columns and so the carriers easily move.
We have confirmed that the front ends of crystals grown like styli or columns have a high nickel concentration similarly to the nickel-doped region. We estimate from this that where devices such as TFTs are fabricated, using these heavily doped regions, the operation of the devices is affected by the nickel. Therefore, neither the starting points of crystals of the crystalline silicon film grown parallel to the substrate nor the end points of the crystal growth are used. It is advantageous to use only the intermediate regions.
Accordingly, in the present invention, as shown in FIGS. 1, (A)-(D), an amorphous silicon film 13 to be crystallized and an overlying silicon oxide film 14 are patterned into islands. A film 15 containing a trace amount of an element such as nickel silicide is formed on the islands. Nickel silicide is formed on the side surfaces 16 of the amorphous silicon film 13. Crystals are caused to grow from these side surfaces as indicated by the arrows 17. Devices such as TFTs are fabricated without using regions 10 and 18 heavily doped with nickel.
That is, neither the starting points of crystals of the crystalline silicon film grown parallel to the substrate nor the end points, or the front end portions, of the crystal growth are used. The intermediate portions are employed, and a crystalline silicon film in which carriers move easily is used. At the same time, regions lightly doped with nickel are used. More specifically, regions lightly doped with nickel can be used by removing (e.g., etching) the regions doped with the metal element for promoting crystallization and the finally grown portions parallel to the substrate after the crystallization.
It is important that the novel crystalline silicon film on a substrate be not a single crystal of silicon. The invention is characterized in that the film is a crystalline silicon film crystallized in the form of a thin film and that the direction of the crystal growth is parallel to the substrate. This film is essentially different from a single crystal of silicon. Therefore, the novel crystalline silicon film can be referred to as a non-single crystal crystalline silicon film.
Elements for promoting crystallization in accordance with the present invention can be selected from the elements belonging to group VIII of the periodic table, i.e., Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt. Also, transition elements Sc, Ti, V, Cr, Mn, Cu, and Zn can be used. Experiments show that Au and Ag promote crystallization. Ni produces especially conspicuous effects among the elements described above. We have confirmed that a silicon film crystallized by the action of Ni is used to fabricate TFTs and that these TFTs operate successfully.
Metal atoms for promoting crystallization are concentrated at the front ends of crystals grown parallel to the substrate. Devices are fabricated in regions located between these front ends and the starting point of growth to which the metal element has been added. Thus, the carriers can be moved at a high speed. At the same time, the concentration of metal elements which are considered to adversely affect movement of the carriers is reduced. Hence, devices having excellent characteristics are obtained.
In another feature of the invention, a non-single crystal semiconductor film (e.g. a silicon film) formed on a substrate is crystallized by heating the film below 600xc2x0 C. and irradiating the film with intense light to enhance the crystallinity. At the same time, the film is made denser.
In a further feature of the invention, a silicon film (e.g. a non-single crystal silicon film) doped with a metal element such as nickel for promoting crystallization is heated to crystallize the film. Then, the film is irradiated with intense light such as infrared light or laser light (e.g., infrared light having a peak at wavelength 1.3 xcexcm) to heat and anneal the film. In this way, the crystallinity is improved.
Elements for promoting crystallization in accordance with the present invention can be selected from the elements belonging to group VIII of the periodic table, i.e., Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt. Also, transition elements Sc, Ti, V, Cr, Mn, Cu, and Zn can be used. Experiments show that Au and Ag promote crystallization. Ni produces especially conspicuous effects among the elements described above. We have confirmed that a silicon film crystallized by the action of Ni is used to fabricate TFTs and that these TFTs operate successfully.
A thin-film silicon semiconductor crystallized by heating below 600xc2x0 C. is irradiated with infrared light or laser light to selectively heat the silicon film. Also, the crystallinity can be enhanced. At this time, the infrared light is not readily absorbed by the glass substrate and so the annealing can be carried out without heating the glass substrate to a large extent.
Other objects and features of the invention will appear in the course of the description thereof, which follows.